Aluminum magnesium titanate is a solid solution which has a complete solubility in the solid state throughout the entire composition region of aluminum titanate-magnesium titanate. A sintered body of the same has as low a coefficient of thermal expansion and as high a corrosion resistance as aluminum titanate sintered bodies. The melting point of aluminum magnesium titanate is, for example, about 1640° C. in a sintered body in which aluminum titanate and magnesium titanate are dissolved into each other at an approximately equivalent molar ratio. This temperature is lower than the melting point of aluminum titanate, about 1870° C., but is higher than the melting point of magnesium titanate, about 1600° C. Therefore, in general, the heat-resistant high temperature limit of aluminum magnesium titanate is lower than that of aluminum titanate, but is higher than that of magnesium titanate.
Aluminum titanate sintered bodies and magnesium titanate sintered bodies are constructed from the crystal grains having the pseudobrookite type crystal structure, and their coefficients of thermal expansion are anisotropic. For this reason, displacement at the crystal grain boundary tends to be caused when heated or cooled and micro cracks and apertures may be formed, leading to disadvantageously lowered mechanical strength. Since aluminum magnesium titanate has similar disadvantages, the strength of its sintered body is also insufficient. Particularly when used for an application in which high temperatures and heavy loads are applied, it cannot exhibit sufficient durability.
The lower limit of the temperature for synthesizing aluminum titanate is 1280° C., while the lower limit of the temperature for synthesizing magnesium titanate is 900° C. Both are unstable in the temperature region lower than their synthesizing temperatures. A solid solution of these compounds, aluminum magnesium titanate is unstable at a temperature lower than the synthesizing temperature region as well as aluminum titanate and magnesium titanate, and thus is tend to thermally decompose into TiO2 (rutile) and MgAl2O4 (spinel) when continuously used in the decomposition temperature range for a long time. Herein, the decomposition temperature range of aluminum magnesium titanate differs depending on the dissolving ratio of aluminum titanate and magnesium titanate, but is normally within the temperature range of about 800–1280° C. For example, a sintered body of aluminum magnesium titanate in which aluminum titanate and magnesium titanate are dissolved at an approximately equivalent molar ratio, when continuously used for a long period in a temperature range of about 1100° C., thermally decomposes into TiO2 (rutile) and MgAl2O4 (spinel) (Vincenzo Buscaglia et al. “Decomposition of Al2TiO5 and Al2(1−x)MgxTi(1+x)O5 Ceramics”, Journal of American Ceramic Society, 1998, 81 [10], pp. 2645–2653). For this reason, a sintered body of aluminum magnesium titanate cannot be continuously used in such a decomposition temperature range.